Chris Carmichael (of _The Time Crunched Cyclist_) built his zone definition system around a pair of 8 minute time trials and finding the required 3-4 mile stretch of road without traffic lights should be a lot easier than finding someplace you can ride for 30 (Friel approximates lactate heart rate off the last 20 minutes of a 30 minute all-out effort) - 60 (the gold standard) minutes.

The results may be more accurate too. Although individual variation in anaerobic work capacity mean the numbers won't be as good as if you did the longer effort at 100% it's a lot easier psychologically and therefore may be closer to a true 100%.